The principles generally applicable to this invention are set forth in Bracewell, R. N. (1956). Strip Integration in Radio Astronomy, Aust. J. Phys. 9, pp. 198-217; Mu, J. P. (1997), Development of a Photopolymer Formulation and Cure Strategy for Inverse Tomographic Construction, Masters Thesis, University of Utah and Thomas C. L., Hayworth, K. (1996) Automating Sheet Based Fabrication, Solid Freeform Fabrication Symposium, Austin, Tex., USA, pp. 281-290. These references will assist those skilled in the art to understand and practice the invention.
Current rapid prototyping (RP) techniques provide important benefits to engineering and industry. These techniques have some practical limitations, however; specifically including construction speed and geometric error due to finite thickness layered construction.
RP techniques may conveniently be classified in terms of their basic construction strategies. While nearly all processes produce prototypes by sequential construction of 2D cross sections, the manner in which the layers are generated divides the many processes into natural classes:
1. Voxel Sequential Volume Addition
Here the term voxel is defined as the smallest unit of volume that can be created by a given device. For stereolithography, this volume would be defined by the layer thickness, the beam width, and the smallest possible step in the scan direction. Voxel sequential processes generate solid geometry one voxel at a time, by scanning a laser line, driving an extruder head, or scanning a droplet deposition head. This concept is modified somewhat by droplet deposition devices with multiple deposition heads. PA1 This class of processes generates each layer all at once. This class of processes includes Solid Ground Curing, which uses a photomask to cure a layer of photopolymer in a single step. PA1 This class of devices are not purely additive devices. Here each layer is cut from a construction sheet and the layers are bonded together to create the prototype in an additive process. Laminated Object Manufacturing is a process belonging to this class. PA1 This class is for processes that operate on the entire construction volume at once to produce the prototype. No existing processes have been identified that fit within this class.
2. Area Sequential Volume Addition
3. Periphery Cutting
4. Volume Sequential Volume Addition
Using this classification scheme, a set of generic equations have been developed for use in comparing the build time required by devices from different classes for parts of varying geometry. (Thomas, 1996) From these equations, it can be concluded that voxel sequential processes are more efficient for thin walled structures, while a periphery cutting device might be preferred for heavy bodied models. Area sequential and volume sequential devices are conceptually independent of part geometry within the layer. While absolute build times depend on the physics of each process, by virtue of the reduced number of steps involved in the area sequential and volume sequential processes may be capable of increasing construction speed.
The layered approach to prototype construction results in a stepped appearance to part surfaces that are not vertical planes in the build direction. This shortcoming can be addressed by making the layers thinner, but the increased number of layers will adversely affect the build speed. While the volume sequential approach does not imply improved surface finish in each instance, a fundamentally different surface results from this process. The advantages and disadvantages of this new surface texture can be assessed once parts have been produced using the volume sequential approach.